Rangefinding, including passive rangefinding, being the estimation of the distance from an observer or observing apparatus to an object or multiple objects of interest or an extended scene is of importance for consumer, military, technical and scientific applications.
Optical rangefinders based on geometrical methods, i.e., operating as angle-measuring devices, are in use since the nineteenth century. In stadimetric systems the distance is calculated, or, alternatively, derived, from the angular size of an object once the dimensions of the object are known a priori (for example, as disclosed in an early prior art document 2,289 from the year 1799), and in parallax/coincidence rangefinders the distance is evaluated from the parallax error of several images of the same object. These devices are passive by nature and their precision is limited by the precision of angular measurements. Passive triangulation methods are also employed in modern photographic cameras (for example, K. Engelhardt and R. Knop, Appl. Opt. 34, 2339-2344, 1995) to estimate the degree of defocus at several locations in the image field.
In modern imaging devices passive rangefinding can be accomplished with phase detection or contrast measurement methods. Phase detection methods (for example, WO2005/098501 and US2008/205872) split the light after the primary objective into several channels with known light-path lengths and analyze the sub-images for differences in light intensity. Contrast measurement methods (for example, US2006109369 and US2004017502) optimize the contrast of the image, or part of the image, on the photosensor by changing the focusing condition.
The above methods differ, in all their aspects, from the rangefinder apparatus and corresponding methods described in the present document which do neither require triangulation or angular measurement of the object nor contrast evaluation or comparison of phase delayed sub-images.